SYSCALL(2)                 Linux Programmer's Manual                SYSCALL(2)

NAME
        syscall - indirect system call

SYNOPSIS
        #include <unistd.h>
        #include <sys/syscall.h>   /* For SYS_xxx definitions */

        long syscall(long number, ...);

    Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
        syscall():
            Since glibc 2.19:
               _DEFAULT_SOURCE
            Before glibc 2.19:
               _BSD_SOURCE || _SVID_SOURCE

DESCRIPTION

        syscall()
        is a small library function
        that invokes the system call
        whose assembly language interface
        has the specified number with the specified arguments.
        Employing syscall()  is useful,
        for example,
        when invoking a system call that
        has no wrapper function in the C library.  

        syscall()
            saves CPU registers before making the system call,
            restores the registers upon return from the system call,
        and stores any error code returned by
        the system call in errno(3)
        if an error occurs. 

        Symbolic constants for system call numbers can be found in the header  file <sys/syscall.h>.

RETURN VALUE

        The return value is defined by
        the system call being invoked.
        In gen‐ eral,
            a 0 return value indicates success.
            A -1 return value indicates an error,
        and an error code is stored in errno. 

NOTES
        syscall() first appeared in 4BSD.

    Architecture-specific requirements 
        Each architecture ABI has its own requirements on how system call argu‐ ments
            are passed to the kernel.
        For system calls that have a glibc wrapper (e.g.,  most system calls),
        glibc handles the details of copying arguments to the right registers
        in a manner suitable for the architec‐ ture.
        However,
        when using syscall()
            to make a system call,
            the caller might need to handle architecture-dependent details;
            this requirement is most commonly encountered on certain 32-bit architectures.  


        For example,
        on the ARM architecture Embedded ABI (EABI),
        a 64-bit value (e.g.,  long long)
        must be aligned to an even register pair.
        Thus,
        using syscall()
        instead of the wrapper provided by glibc,
        the readahead()  system call
        would be invoked as follows on the ARM architecture with the EABI in little endian mode: 

            syscall(SYS_readahead, fd, 0,
                   (unsigned int) (offset & 0xFFFFFFFF),
                   (unsigned int) (offset >> 32),
                   count);

        Since the offset argument is 64 bits, and the first argument (fd) is
        passed in r0, the caller must manually split and align the 64-bit value
        so that it is passed in the r2/r3 register pair. That means inserting
        a dummy value into r1 (the second argument of 0). Care also must be
        taken so that the split follows endian conventions (according to the C
        ABI for the platform).

        Similar issues can occur on MIPS with the O32 ABI, on PowerPC and
        parisc with the 32-bit ABI, and on Xtensa.

        while the parisc C ABI also uses aligned register pairs, it
        uses a shim layer to hide the issue from user space.

        The affected system calls   are   fadvise64_64(2),   ftruncate64(2),
        posix_fadvise(2),      pread64(2),      pwrite64(2),      readahead(2),
        sync_file_range(2), and truncate64(2).

        This does not affect syscalls that manually split and assemble 64-bit
        values such as _llseek(2), preadv(2), preadv2(2), pwritev(2), and
        pwritev2(2). Welcome to the wonderful world of historical baggage.

    Architecture calling conventions 
        Every architecture has its own way of invoking and
        passing arguments to the kernel.
        The details for various architectures are listed in the two tables below. 

        The first table lists the instruction used to transition to kernel mode
        (which might not be the fastest or best way to transition to the ker‐
        nel, so you might have to refer to vdso(7)), the register used to indi‐
        cate the system call number, the register(s) used to return the system
        call result, and the register used to signal an error.

        Arch/ABI    Instruction           System Ret Ret Error    Notes
                                         call # val val2
        ───────────────────────────────────────────────────────────────────
        alpha       callsys               v0      v0   a4   a3       1, 6
        arc         trap0                 r8      r0   -    -
        arm/OABI    swi NR                -       a1   -    -        2
        arm/EABI    swi 0x0               r7      r0   r1   -
        arm64       svc #0                x8      x0   x1   -
        blackfin    excpt 0x0             P0      R0   -    -
        i386        int $0x80             eax     eax edx -
        ia64        break 0x100000        r15     r8   r9   r10      1, 6
        m68k        trap #0               d0      d0   -    -
        microblaze brki r14,8            r12     r3   -    -
        mips        syscall               v0      v0   v1   a3       1, 6
        nios2       trap                  r2      r2   -    r7
        parisc      ble 0x100(%sr2, %r0) r20     r28 -    -
        powerpc     sc                    r0      r3   -    r0       1
        powerpc64   sc                    r0      r3   -    cr0.SO   1
        riscv       ecall                 a7      a0   a1   -
        s390        svc 0                 r1      r2   r3   -        3
        s390x       svc 0                 r1      r2   r3   -        3
        superh      trap #0x17            r3      r0   r1   -        4, 6
        sparc/32    t 0x10                g1      o0   o1   psr/csr 1, 6
        sparc/64    t 0x6d                g1      o0   o1   psr/csr 1, 6
        tile        swint1                R10     R00 -    R01      1
        x86-64      syscall               rax     rax rdx -        5
        x32         syscall               rax     rax rdx -        5
        xtensa      syscall               a2      a2   -    -

        Notes:

        [1] On a few architectures, a register is used as a boolean (0 indicat‐
            ing no error, and -1 indicating an error) to signal that the system
            call failed. The actual error value is still contained in the re‐
            turn register. On sparc, the carry bit (csr) in the processor sta‐
            tus register (psr) is used instead of a full register.   On pow‐
            erpc64, the summary overflow bit (SO) in field 0 of the condition
            register (cr0) is used.

        [2] NR is the system call number.

        [3] For s390 and s390x, NR (the system call number) may be passed di‐
            rectly with svc NR if it is less than 256.

        [4] On SuperH, the trap number controls the maximum number of arguments
            passed. A trap #0x10 can be used with only 0-argument system
            calls, a trap #0x11 can be used with 0- or 1-argument system calls,
            and so on up to trap #0x17 for 7-argument system calls.

        [5] The x32 ABI shares syscall table with x86-64 ABI, but there are
            some nuances:

            • In order to indicate that a system call is called under the x32
              ABI, an additional bit, __X32_SYSCALL_BIT, is bitwise-ORed with
              the system call number. The ABI used by a process affects some
              process behaviors, including signal handling or system call
              restarting.

            • Since x32 has different sizes for long and pointer types, lay‐
              outs of some (but not all; struct timeval or struct rlimit are
              64-bit, for example) structures are different. In order to han‐
              dle this, additional system calls are added to the system call
              table, starting from number 512 (without the __X32_SYSCALL_BIT).
              For example, __NR_readv is defined as 19 for the x86-64 ABI and
              as __X32_SYSCALL_BIT | 515 for the x32 ABI. Most of these addi‐
              tional system calls are actually identical to the system calls
              used for providing i386 compat. There are some notable excep‐
              tions, however, such as preadv2(2), which uses struct iovec en‐
              tities with 4-byte pointers and sizes ("compat_iovec" in kernel
              terms), but passes an 8-byte pos argument in a single register
              and not two, as is done in every other ABI.

        [6] Some architectures (namely, Alpha, IA-64, MIPS, SuperH, sparc/32,
            and sparc/64) use an additional register ("Retval2" in the above
            table) to pass back a second return value from the pipe(2) system
            call; Alpha uses this technique in the architecture-specific getx‐
            pid(2), getxuid(2), and getxgid(2) system calls as well. Other ar‐
            chitectures do not use the second return value register in the sys‐
            tem call interface, even if it is defined in the System V ABI.

        The second table shows the registers used to pass the system call argu‐
        ments.

        Arch/ABI      arg1 arg2 arg3 arg4 arg5 arg6 arg7 Notes
        ──────────────────────────────────────────────────────────────
        alpha         a0    a1    a2    a3    a4    a5    -
        arc           r0    r1    r2    r3    r4    r5    -
        arm/OABI      a1    a2    a3    a4    v1    v2    v3
        arm/EABI      r0    r1    r2    r3    r4    r5    r6
        arm64         x0    x1    x2    x3    x4    x5    -
        blackfin      R0    R1    R2    R3    R4    R5    -
        i386          ebx   ecx   edx   esi   edi   ebp   -
        ia64          out0 out1 out2 out3 out4 out5 -
        m68k          d1    d2    d3    d4    d5    a0    -
        microblaze    r5    r6    r7    r8    r9    r10   -
        mips/o32      a0    a1    a2    a3    -     -     -     1
        mips/n32,64   a0    a1    a2    a3    a4    a5    -
        nios2         r4    r5    r6    r7    r8    r9    -
        parisc        r26   r25   r24   r23   r22   r21   -
        powerpc       r3    r4    r5    r6    r7    r8    r9
        powerpc64     r3    r4    r5    r6    r7    r8    -
        riscv         a0    a1    a2    a3    a4    a5    -
        s390          r2    r3    r4    r5    r6    r7    -
        s390x         r2    r3    r4    r5    r6    r7    -

        superh        r4    r5    r6    r7    r0    r1    r2
        sparc/32      o0    o1    o2    o3    o4    o5    -
        sparc/64      o0    o1    o2    o3    o4    o5    -
        tile          R00   R01   R02   R03   R04   R05   -
        x86-64        rdi   rsi   rdx   r10   r8    r9    -
        x32           rdi   rsi   rdx   r10   r8    r9    -
        xtensa        a6    a3    a4    a5    a8    a9    -

        Notes:

        [1] The mips/o32 system call convention passes arguments 5 through 8 on
            the user stack.

        these tables don't cover the entire calling convention—some
        architectures may indiscriminately clobber other registers not listed
        here.

EXAMPLE
        #define _GNU_SOURCE
        #include <unistd.h>
        #include <sys/syscall.h>
        #include <sys/types.h>
        #include <signal.h>

        int
        main(int argc, char *argv[])
        {
            pid_t tid;

            tid = syscall(SYS_gettid);
            syscall(SYS_tgkill, getpid(), tid, SIGHUP);
        }

SEE ALSO
        _syscall(2), intro(2), syscalls(2), errno(3), vdso(7)

COLOPHON
        This page is part of release 5.05 of the Linux man-pages project. A
        description of the project, information about reporting bugs, and the
        latest     version     of     this    page,    can    be    found    at
        https://www.kernel.org/doc/man-pages/.

Linux                             2020-02-09                        SYSCALL(2)
